We are developing a computer based quantitative decision aiding methodology to assist physicians during the chemotherapy of leukemia. Our methods use mathematical models, computer simulations, and data analysis to formalize existing relevant data on the biology of the disease, the physiology of the bone marrow, and the action of the drugs. Our system, when completed, could be used to identify critical decision points in therapy, to quantify the risk and benefit to the patient of medical alternatives, and to provide the optimum treatment plan to coordinate clinical practice and the major cancer centers. Our system is modular: the first module is the leukemia/therapy simulator where cell kinetics, pharmacokinetics, the patient's data on blood counts and prior therapy, and marrow mechanics are all used to predict the response of the patient to anti-tumor drugs and schedules. Other modules, to be developed later, deal with the prediction of complications, the effects of organ toxicity, the activity status of the patient, and the development of optimal scheduling strategies.